Abstract : The thermal-optical method has been widely used for quantifying total carbon TC, organic carbon OC, and elemental carbon EC content in ambient and source particulate samples. Thermally defined carbon fractions have been used for source identification. Temperature precision in thermal carbon analysis is critical to the allocation of carbon fractions. The sample temperature is determined by a thermocouple, which is usually located in the oven near the sample punch, and which may be biased due to different thermal properties between the sample filter punch and the thermocouple, or the inhomogeneity in the heating zone. Quick-drying temperature-indicating liquids Tempil Inc., South Plainfield, NJ of different liquefying points are used as temperature calibration standards. These temperature indicators consist of chemicals that change their appearance at specific temperatures, and can be optically monitored to determine the sample temperature. Temperature measures were evaluated for three different models of carbon analyzers. The actual sample temperature was frequently underestimated by 10 to 50°C in all three analyzers. This experiment allowed calibration curves to be constructed that relate the sample temperature to the temperature measured by a thermocouple. Even though temperature variations of up to 50°C do not alter the OC and EC concentrations, a positive bias of 14 to 22°C during thermal analysis can significantly change carbon fraction measurements.